Shop Solutions: Machining Features that Aren't Really There

ETJ Corp. (Austin, TX) is a small precision manufacturing company that is having good success in using CAM to develop manufacturing processes to machine replacement wing flap tracks for the US Air Force’s 40 year-old, C-5 transport plane. Within recent months, the company has submitted first articles and the required certified process documentation for three versions of the flap tracks. Two of them have been approved and approval is expected on the third configuration. These approvals will make ETJ one of four or five companies that have been approved to manufacture these deceptively simple-looking parts.

Efren Bermudez, ETJ engineering manager, said, “Making parts for the DOD is unlike any other type of manufacturing. You have to acquire certified materials that are available from a limited number of suppliers; you have to document every step of your manufacturing process; you even have to document your shipping procedures. That is half the battle. The other is being able to actually make the part, which is difficult in this case because inherent stresses in the material can distort the part as you are making it, unless you know exactly what you are doing and are very careful.”The flap tracks are shaped like a hockey stick and can be up to 130" (3.3-m) long. To make them, ETJ purchased a four-axis Vision-Wide machining center with a 123 × 63 × 35" (3124 × 1600 × 889-mm) table. The longest flap tracks are fixtured on the table diagonally. Bermudez uses Mastercam from CNC Software Inc. (Tolland, CT) to model and develop manufacturing processes to make these parts. This software, which the company has been using since it first acquired CNC equipment more than 20 years ago, gives Bermudez all kinds of options for visualizing what needs to be done to make the part and for writing toolpaths that will accomplish specific objectives at each stage of manufacturing.

“Writing the Mastercam program was the easy part,” said Bermudez. “Once I figured out what needed to be accomplished with a specific operation, writing the CAM program typically took less than an hour. The hard part was figuring out the right sequence of operations so that we could ultimately make a part that would comply with the Lockheed Martin specifications. That thought process took place over two months to get the CAM process right for just one part.”

From having manufactured parts similar to these in the past, Bermudez understood what areas of the part would distort if manufactured incorrectly. These predictable problems were a function of the part’s slender geometry. He also knew that other problems would arise due to stresses that would be revealed in the part’s forged aluminum workpiece as it was being machined. These have to be detected as they occur by checking the first article frequently, either on the machine or a nearby CMM.

Once these stresses and deformations revealed themselves, Bermudez compensated for them by altering the machining strategy in Mastercam at a particular manufacturing stage.

“A big mistake some manufacturers make in attempting to write a program for a part like this is creating the model and then using their CAM software to create a single program to make it. That would be easy, but it doesn’t work that way. What you actually have to do is write programs to make the part in six or seven stages. You are actually creating maybe six parts to finally get to the one you want that is largely stress free. Those intermediate parts may look very different from the one you finally end up with.”

Bermudez would not disclose what these intermediate parts looked like. ETJ considers those to be proprietary knowledge. However, he was willing to talk about the thought processes and CAM strategies he uses during various stages of manufacturing:

How to Hold It: The flap track is a part with a lot of deep details and the walls are all tapered on the outside. So the first thing that has to be considered in making this is how it’s going to be held securely while allowing tools to reach critical areas. Bermudez uses the CAD features of Mastercam to design a workholding solution for the part in its finished form and its preceding iterations. He can work back and forth between the CAD and CAM views to assure that the holding solution he designs will afford appropriate clearances for tools at every stage.

Knowing the Weaknesses: “The key is knowing how you are going to remove material without generating any more stresses than are already in there,” Bermudez said. Here is where experience comes into play as he determines what features of the part might be compromised if machined in the wrong sequence, too aggressively, or by removing supporting materials prematurely.

Waterline Toolpath: ETJ relies heavily on Mastercam’s Waterline toolpath that removes material at the same Z-axis height all the way around the part, with the tool dropping down in small increments after each trip around the part. The part takes shape from the top down as if it were sitting in a tub of water that is gradually lowering to reveal its dimensions. This approach keeps stresses balanced because cutting is always at the same level.

Simulation: As Efren programs, he frequently uses Mastercam’s simulation features, Backplot and Verify, to review tool motions and material removal in critical areas. Sometimes he can see that more material needs to be left behind for subsequent operations. For example, he might see an inadequate surface finish and decide that it would be better to leave more material in a certain location so that he can come back and put a fine finish on it with Mastercam’s Flow Line toolpath.Hedging Against Scrap: The workpiece forgings for these parts are expensive, up to $3500 apiece. As of this date, ETJ has manufactured three different configurations of C5 flap tracks and has secured DOD approvals for them without scrapping a single forging. Bermudez hedges against scrap by programming the initial cuts high in the forging, leaving plenty of material below it. When he discovers unexpected stresses during his iterative development process, he can regroup and create a different material removal strategy a little deeper in the part.

The manufacturing of a C-5 wing flap track first article at ETJ has taken up to 80 hours of CNC cutting time with even more than 80 hours spent performing in-process inspections to assure dimensional integrity. That care has paid off because the company has been approved to manufacture three members of this part family and one other has been submitted for approval.

Bermudez said there at are least 40 more versions of this replacement component and ETJ hopes to be selected to manufacture some or all of them. His company takes the replacement component business very seriously because it is critical to keeping older but still highly functional military vehicles and aircraft in good operating condition. He concluded that small companies like his, with demonstrated expertise in developing these specialized manufacturing processes, are important resources that fill a critical need. ME

For more information from Mastercam/CNC Software Inc., go to www.mastercam.com, or phone 860-875-5006.

This article was first published in the March 2014 edition of Manufacturing Engineering magazine. Click here for PDF.